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WASHINGTON - Federal regulators are telling airlines they can fly Boeing's 787 Dreamliners again as soon as they replace its problematic lithium ion batteries with a revamped battery system.

A Federal Aviation Administration safety order posted online Thursday applies to all U.S. airlines, but only one airline -- United -- currently has 787s in their fleet. They have six. The FAA estimated the repair costs for those planes at $2.8 million.

The plane is important to Denver's economy, because United plans to use the 787 for a non-stop flight from Denver International Airport to Tokyo. Denver mayor Michael Hancock said last year that the new flight will generate more than $130 million in annual economic benefit to Colorado.

The flight was initially scheduled to begin in March. The start date has been delayed to June 10.

The 787's have been grounded since mid-January, following a battery fire on a 787 parked at Boston's Logan International Airport, and a smoking battery that led to an emergency landing by another 787 in Japan.

There are 50 of the planes in service worldwide, but Boeing has purchase orders for 840 more planes. Newly delivered planes will come with the revamped system.

Copyright 2013 Scripps Media, Inc. The Associated Press contributed to this report. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.